1. Field of the Invention
The present invention relates to optical circuit elements used mainly in the fields of optical communication and sensors, and more particularly, to a lensed optical fiber coupled to a semiconductor laser with high efficiency and a laser module.
2. Description of the Prior Art
A technique for coupling a semiconductor laser and an optical fiber with high efficiency is one of the most important techniques in the field of optical communication. Conventionally, for example, there is a method that uses a lens such as a spherical or nonspherical lens. Another method is based on the use of a hemispherically-ended fiber that has a spherical distal end.
Although the method that uses the lens can enjoy a relatively high coupling efficiency, it entails troublesome alignment between the semiconductor laser, lens, and optical fiber, and requires a coupling system of a substantial size. Further, the lens used in this method is so bulky and occupies so wide a space that the method cannot be applied to coupling between a semiconductor laser array and an optical fiber array, in which a plurality of semiconductor lasers or optical fibers are arranged at short intervals.
On the other hand, the method based on the use of the hemispherically-ended fiber is applicable to coupling between laser and fiber arrays, since the hemispherically-ended fiber is small-sized. FIG. 8 shows the simplest structure of the hemispherically-ended fiber. In a hemispherically-ended fiber 1 shown in FIG. 8, a hemispherical lens portion 1c is formed integrally on the distal end of a single-mode fiber that includes a core 1a and a cladding 1b.
In order to couple the hemispherically-ended fiber 1 and the semiconductor laser without lowering the coupling efficiency to an extreme degree, the radius of curvature of the lens portion 1c and the distance between the lens portion 1c and the end face of the semiconductor laser, i.e., working distance, must be adjusted to about 10 .mu.m, which is equivalent to the core diameter of the fiber 1. In assembling the coupling system that combines the hemispherically-ended fiber 1 and the semiconductor laser, therefore, the laser and the fiber 1 may run against one another, thereby getting out of order.
Accordingly, there have already been proposed some means for elongating the working distance of the hemispherically-ended fiber.
For example, there is a known optical fiber in which a coreless fiber, having a uniform refractive index and a spherical distal end, is connected integrally to the distal end of a single-mode fiber. With this optical fiber, the working distance can be set at 100 .mu.m or more, and the collision between the laser and the fiber can be avoided.
In the optical fiber having the coreless fiber connected to the distal end of single-mode fiber, the distal end of the coreless fiber is spherical, so that the coupling efficiency is lowered by spherical aberration. More specifically, light beams that are emitted from the laser end face reach the end face of the single-mode fiber in varied positions and at different angles, depending on their angles of emission. Therefore, some light beams may fail to reach the core or may reach the core wider than the critical angle at angles of incidence, thus failing to become ones that propagate through the single-mode fiber, so that the coupling efficiency is lowered. For example, the coupling loss is at 6 dB or thereabout if the semiconductor laser used is a standard one.
In order to solve the above problem, there is proposed a lensed fiber (e.g., U.S. Pat. No. 5,774,607, etc.) in which one end of a single-mode fiber that includes a core and a cladding and the other end of a coreless fiber are connected by means of a square-law index fiber that has a square-low index distribution of a length equal to 1/4 of the meandering period of propagative light or an odd multiple thereof.
FIG. 9 shows a lensed fiber 5 that is proposed as a simplified version of the aforementioned lensed fiber. In this fiber 5, a square-law index fiber 7 that has a square-low index distribution of a length equal to 1/4 of the meandering period of propagative light or an odd multiple thereof is connected to a single-mode fiber 6 that includes a core 6a and a cladding 6b. The square-low index fiber 7, which includes a core 7a and a cladding 7b, has a hemispherical distal end.
When these two different optical fibers were coupled to a semiconductor laser, the coupling loss was reduced to about 4 dB or thereabout, which does not comply with the practical requirement, 3 dB or less.
In general, the smaller the coupling loss between a semiconductor laser and an optical fiber, the higher the performance of an optical communication system is, and the easier the construction of the system is.
However, the technique associated with the conventional lensed optical fibers, including the lensed fiber 5 shown in FIG. 9, cannot concurrently fulfill the two requirements, maintenance of a long working distance and reduction of the coupling loss.